The present invention relates a method for identifying anti-parasitic compounds. More specifically, the present invention relates to a method for the identification of compounds capable of binding and/or inhibiting cyclophilin-like proteins, as well as to methods of treating parasitic infections which are not susceptible to cyclosporin A.
Cyclophilin is a common protein, which by definition, binds avidly to the immunosuppressive agent cyclosporin A (CsA). CsA is a fungal cyclic undecapeptide, which at present is a widely used therapeutic agent for the prevention of graft rejection. This drug is therefore preferentially used in kidney, liver, heart and bone marrow transplantation, and in the treatment of various autoimmune diseases [Kahn, Cyclosporin: Biological Activity And Clinical Applications Grune & Stratton, Orlando, FL (1983)].
Cyclophilin has recently been shown to posses peptidyl-prolyl cis-trans isomerase (PPiase) or rotamase activity [Fischer, et al., Nature, 337:476-478 (1989)], and CsA has been demonstrated to actively inhibit this enzymatic activity [Takahashi, et al, Nature, 337:473-475 (1989)]. This enzyme catalyzes the cis-trans isomerization of proline-imidic peptide bonds in oligopeptides and has been demonstrated to accelerate the refolding of several proteins, including collagens [Bachinger, J. Biol. Chem., 262:17144-17148 (1987)]. In addition to actively inhibiting PPiase activity, [Takahashi, et al., Nature, 337:473-475 (1989)]CsA has been demonstrated to slow down the in vitro folding of collagen triple helices [Steinmann, et al., J. Biol. Chem., 266:1299-1303 (1991)]. Not all cyclophilins bind CsA to the same degree. In a study involving E. coil and human cyclophilins, it has been clearly shown that the major determinant in the binding of CsA by cyclophilin is a tryptophan residue in the drug binding domain [Lui, et al., Biochemistry, 30:2306-2310 (1991)]. It also been shown that cyclophilins in which this tryptophan residue has been substituted by another amino acid will not bind to CsA [(Kieffer, et al., J. Biol. Chem. 267:5503-5507 (1992)].
Investigators have only recently been able to elucidate the mode of action of both CsA, and the functionally, but not chemically related immunosuppressant FK-506 on T-cells. The PPiase activity of both cyclophilin and the FK-506 receptor FKBP are now not believed to be involved in its immunosuppressive action. It is currently hypothesized that CsA and FK-506 bind to endogenous cytosolic cyclophilin or FKBP to form a complex which can subsequently bind to calcineurin, therefore inhibiting dephosphorylation and preventing access to transcription factors such as NF-AT into the nucleus of the T-cell [Schreiber & Crabtree, Immunol. Today, 13:136-142 (1992)].
CsA has also been demonstrated as having broad spectrum anti-parasite effects [Chappell & Wastling, Parasitol., 105:S25-S40 (1992)]. Parasitic protozoa affected, include Leishmania major [(Behforouz, et al, J. Immunol, 136:3067-3075 (1986)]and Plasmodium species [Nickell, et al., Infect. & Immunol., 37:1093-1100 (1982); Thommen-Scott, Agents & Actions, 11:770-773 (1981)]. Susceptible helminth parasites include the trematode parasites Schistosoma mansoni [(Nilsson, et al, Parasitol. Immunol., 7:19-27 (1985); Pons, et al, Exper. Parasitol, 67:190-198 (1988); Munro & McLaren, Parasitol., 100:19-29 (1990a) and Munro & McLaren, Parasitol., 100:29-34 (1990b)]and Paragonimus miyazakii [Hashiguchi & Okamura, J. Helminthol, 62:251-256 (1988)], the cestode species Hymenolepis microstoma [Wastling, et al., Parasitol., 104:531-538 (1992)]. Nematode species affected by CsA include Acanthocheilonema vitae [Bout, et al., Trans. Roy. Soc. Trop. Med. Hyg., 78:670-671 (1984)], Litomosoides carinii [Zahner & Schultheiss, J. Helminthol., 61:282-290 (1987)]and Trichinella spiralis [Bolas-Fernandez, et al, Parasit. Immunol., 10:111-116 (1988)]. In one example, CsA administered to the host at sub-immunosuppressive levels prior to S. mansoni infections, was demonstrated as exerting profound Schistosomicidal effects, causing gross herniation of the parasites gut and blistering of the tegumental surface [Munro & McLaren, Parasitol., 100:19-29 (1990a)]. These effects have also been demonstrated in vitro with both S. mansoni and Fasciola hepatica [Chappel, et al., Parasitology, in press (1993)], therefore ruling out the possibility of CsA exerting an indirect effect via the host. Interestingly cyclophilin has been identified in S. mansoni [Koletsky, J. Immunol., 137:1054-1059 (1986)], and has recently been cloned from the closely-related trematode S. japonicum [Argaet & Mitchell, J. Parasitol., 78:660-664 (1992)]using a probe corresponding to the cyclophilin gene from the cestode parasite Echinococcus granulous [Lightowlers, et al., Mol. Biochem. Parasitol., 36:287-290 (1989)].
CsA anti-nematode effects include the reduction in the microfilarial levels in rodents infected with L. carinii [Zahner, et al., J. Helminthol., 61:282-290 (1987)], killing of A. vitae in rodents [Bout, et al., Trans Roy. Soc. Trop. Med. Hyg., 78:670-671 (1984)]. In the adenophorean nematode T. spiralis, treatment of infected mice with this drug resulted in a significant reduction of muscle stage larvae [Bolas-Fernandez, et al., Parasit. Immunol., 10:111-116 (1988)]. In common between the anti-nematode effects of this drug are its selective effects against early larval stages, with general resistance of reduced susceptibility in the later adult stages.
However, not all parasites have been found to be susceptible to the effects of CsA. For example, in Brugia pahangi, CsA showed no effect on either adults or microfilarial levels [Lawrence et al., Parasit. Immunol. 14:371(1992)]. It would therefore be desirable to have a compound that could be used to treat parasites which are not susceptible to the anti-parasitic effects of CsA.
It would be desirable to have a method which can be used to readily screen and select compounds that are capable of binding cyclophilins from parasites which are not susceptible to the anti-parasitic effects of CsA and/or which inhibit the PPiase activity of such proteins. More specifically, it would be desirable to have a method which can be used to screen and select CsA derivatives that are capable of binding such cyclophilins and inhibiting PPiase activity while having reduced immunosuppressive activity on the host.